This application claims the priority of German Patent Document No. 197 19 998.4, filed in Germany on May 13, 1997, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a process and a device for reducing nitrogen oxides contained in the exhaust gas of a combustion device by the addition of a reducing agent.
Processes and devices of this type are customary, for example, in motor vehicle engines. The air excess on a diesel engine that is operated in a quality-controlled manner, as well as the lean operation that is frequently used in the case of Otto engines for reducing the fuel consumption, prevent the nitrogen oxides emitted by the engine from being converted by other than catalytic reduction. As a remedy, it is known to add a suitable reducing agent to the exhaust gas to be purified with respect to the nitrogen oxides.
Thus, it is suggested in the Published German Patent Application DE 40 38 054 A1 to evaporate an aqueous urea solution in an evaporator and in the process hydrolyze ammonia and carbon dioxide. The hydrolysis is optionally completed by a hydrolysis catalyst.
The gas mixture of ammonia and water formed by the urea hydrolysis at 140.degree. to 300.degree. C. is injected as the reducing agent into the exhaust gas of a combustion device, such as a motor vehicle combustion engine. By means of a reduction catalyst, a selective catalytic reduction of the nitrogen oxides will then take place.
Furthermore, German Published Patent Application DE 42 03 807 A1 discloses constructing an evaporator as a flow mixer and, as a result, permits a compact construction of the nitrogen oxide reducing device. This reference also discloses adding the aqueous urea solution by means of a corresponding control as a function of the rotational engine speed and of the engine load in order to keep a reduction catalyst free of excessive ammonia. In this case, the urea addition is activated only when the exhaust gas temperature, which is sensed at several points by means of corresponding sensors, is above a definable limit value. However, during the injecting of a urea solution (which is preheated as required) directly into an exhaust gas pipe in front of the reduction catalyst, undesirable reaction products may form. The urea is decomposed by thermolysis, while developing gas, to a remeltable cyanuric acid if it is heated above its melting point. When urea is heated between 130.degree. to 250.degree. C., a sublimation would occur under a partial decomposition to ammonia and the reactive isocyanic acid, while biuret (i.e., carbamylurea), the cyanuric acid and ammelide would remain in the residue. The cyanuric acid and ammelide, as well as possibly additional substances, would be a considerable hindrance for a gasification of the urea because the solid urea would first result in liquid urea and then solid, unmeltable substances would again be formed.
U.S. Pat. No. 5,296,206 discloses the injecting of a reducing agent formed by the evaporation of an aqueous urea solution or ammonia solution into the exhaust gas flow of a combustion device for the purpose of a catalytic, or as an alternative, also non-catalytic reduction of nitrogen oxides contained in the exhaust gas. In the disclosed device, the evaporator is heated by a compressed-air flow which previously had been guided via a heat exchanger situated in the exhaust gas flow and is therefore capable of absorbing exhaust gas heat. The reducing agent injection operation is carried out by a corresponding control as a function of respective relevant operating parameters, such as the exhaust gas flow rate, the exhaust gas temperature and the nitrogen oxide concentration of the exhaust gas.
In European Published Patent Application EP 0 615 777 A1, a process and a device are described for the selective catalytic nitrogen oxide reduction in the exhaust gas of a combustion device, such as an internal-combustion engine. Solid urea is added to the exhaust gas as the reducing agent. In this case, the urea is fed by means of compressed air in the form of microprills, which have a grain diameter of between 10 .mu.m and 1,000 .mu.m, and is atomized into the exhaust gas flow. For maintaining the flowability of the hygroscopic urea microprill, special measures are required.
The present invention is based on the technical problem of providing a process and a device of the initially mentioned type by means of which the nitrogen oxides contained in the exhaust gas of a combustion device can be reduced comparatively simply and reliably.
The present invention solves this problem by providing (1) a process comprising melting solid pure urea to form a molten product, and adding the molten product to the exhaust gas as the reducing agent for the reduction of nitrogen oxides; and (2) a device comprising a liquefaction and injection device for heating solid pure urea above its melting temperature to obtain a molten product and for the controlled injection of the molten product into the exhaust gas of the combustion device as a reducing agent for the reduction of nitrogen oxides.
According to the process of the present invention, a molten product is added as the reducing agent for the reduction of nitrogen oxides. The molten product is obtained by the melting of pure urea by means of heating the urea. As in the case of all reducing agent additions based on urea, the ammonia that is formed from the urea is also used as the primary nitrogen oxide reducing agent. By means of the thermal pretreatment of the urea (i.e., its liquefaction), a pyrolysis of urea to ammonia and possibly to isocyanic acid can take place so that the molten product added to the exhaust gas already contains ammonia. By adding this reducing agent in liquid form to the exhaust gas, the known difficulties connected with the evaporation of aqueous urea solutions or the entering of urea in a solid, finely pulverized form are avoided.
According to another embodiment of the present invention, the heating of the pure urea during the melting operation takes place at a temperature of no more than approximately 250.degree. C., preferably at a temperature of between 133.degree. C. and approximately 160.degree. C. At a typical heating rate of, for example, 20.degree. C. per minute, the formation of undesirable urea decomposition products, for example, cyanuric acid from the trimerization process of isocyanic acid, which is promoted by higher temperatures, can therefore be prevented.
A device according to the present invention that is suitable for carrying out the process includes a liquefaction and injection device by means of which the pure urea can be heated in a controlled manner above the melting temperature. The thus obtained molten product can be injected into the exhaust gas flow of the combustion device as the reducing agent for the reduction of nitrogen oxides. For example, in the case of an internal-combustion engine, the injection can take place in the engine's combustion spaces or in an exhaust gas line.
A device according to another embodiment of the present invention is designed for use in the case of an internal-combustion engine and includes a liquefaction and injection device having a heating apparatus, a conveying apparatus and an injection apparatus. The conveying apparatus feeds a quantity of urea, which can be controlled as a function of the engine load and of the rotational engine speed, to the heating apparatus. The injection apparatus carries out the injecting of the molten product provided by the heating apparatus as a function of the engine load, the rotational engine speed and the crank angle of the internal-combustion engine.
In another embodiment of the present invention, the heating apparatus may contain an exhaust gas heat exchanger by means of which heat from a controllable portion of the exhaust gas flow is utilized for the liquefaction of the urea.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.